A light control element such as a resonant type optical modulator is used in a transmission device of an optical communication system such as an optical modulator for optical transmission of a high frequency signal of several GHz or more used for wireless communication, or a pulsar modulator for generating optical clocks used for long-haul transmission along with data modulation. A substrate material having an electro-optic effect such as lithium niobate is used for the resonant type optical modulator, and the intensity or a phase of light which propagates through an optical waveguide is modulated by varying a refractive index of the optical waveguide formed on the substrate using a control electrode having a resonant type electrode.
In the resonant type electrode, when an electric signal with a characteristic frequency is input from a feeding point thereto, a standing wave of the electric signal arises in the electrode. As such, since the resonant type optical modulator uses a resonance of an electric signal, it can be operated with high efficiency when a characteristic frequency is input thereto, and generally has higher modulation efficiency per unit length of electrode than a traveling wave type optical modulator.
With these characteristics, there are many cases where the length of an electrode of a resonant type optical modulator in the related art is designed to be shorter than one wavelength of an electric signal. However, under a condition in which the velocity of light propagating through the optical waveguide nearly matches the velocity of a control signal propagating through the electrode of an interaction regions, the electrode can be extended, and a driving voltage can be reduced proportional to the length of the electrode, within limitation of attenuation of the control signal.
NPL 1 discloses that using velocity matching and the resonant type electrode together is useful for improving efficiency, and, NPL 2 discloses a resonant electrode type optical modulator where lithium niobate is used for a substrate, and discloses a case where favorable characteristics are obtained by setting the refractive index (nm) of an electric signal to nearly 2.2 (the refractive index of lithium niobate to light is about 2.2).
On the other hand, under a condition in which velocities of the light and the electric signal do not match each other, the length of the electrode cannot be sufficiently extended, and, as a result, the total modulation efficiencies cannot be improved even if modulation efficiency per unit length is high. For this reason, a half wavelength voltage Vπ which is a parameter indicating efficiency of a modulator becomes approximately 10 V or more, and a sufficient operation cannot be achieved unless a very high voltage is applied.
In addition, a portion of the optical waveguide is branched into two paths so as to have a Mach-Zehnder (MZ) interferometer structure, which is operated as a light intensity modulator if two branched lightwaves are made to interfere with each other. In a pulsar modulator or the like for generating optical clocks used for long-haul transmission along with data modulation, a phase shift amount of light in the two branched light beams is preferably in a state where wavelength chirp is zero that is obtained by driving an operation through phase shifts of reverse signs with the same magnitude, and a signal of a reverse sign with the same magnitude is applied to each branched waveguide of the MZ interferometer. In addition, there is an effect of decreasing a driving voltage in a case of applying signals of reverse signs to each other to dual-electrodes corresponding to the respective branched waveguides (also referred to as a “dual-electrode type”).
For this reason, if a dual-electrode type MZ modulator is formed using a long resonant type electrode where the velocities of the light and the control signal match each other, dramatic reduction of driving voltage is expected. However, the resonant type electrode has high efficiency and a strong electric field of a signal at a resonant frequency, and is strongly coupled (crosstalk of signals) with a peripheral conductive substance. The phase of the control signal is disturbed, and thereby a desired optical modulation cannot be obtained, when signals of both the control electrodes are in a crosstalk state.
Particularly, a signal tends to be coupled with an element (component) which satisfies a resonance condition with respect to the same frequency. In relation to the dual-electrode type MZ optical modulator, electrodes having the same basic structure are used in the control electrodes corresponding to the respective branched waveguides. For this reason, in a case of forming resonant type electrodes, both of the respective control electrodes (resonant type electrodes) have a resonant frequency of the same frequency and thus the coupling (crosstalk of signals) becomes remarkable. In addition, in a case of using a long electrode in which velocity matching is made, since the electrode is long, influence of the coupling between both electrodes becomes more remarkable.
In relation to a method of preventing crosstalk of control signals between a plurality of electrodes in a case of an optical modulator which is manufactured by forming coplanar electrodes on a substrate made of lithium niobate having an electro-optic effect, there is an example in which a groove is formed as in PTL 1. Generally, the intensity of a control electric field becomes smaller according to an increase in the distance from the control electrode. This is a phenomenon depending on even a structure of a substrate or an electrode; however, there is a disclosure that, as shown in FIG. 1, the strength of an electric field applied to the other optical waveguide is about 1% in a case where a gap between the waveguides is about 150 μm, about 0.2% in a case of about 300 μm, and about 0.1% in a case of about 400 μm.
As such, the wide gap between the two branched optical waveguides of the MZ interferometer is disadvantageous in terms of the size of a device or costs. In addition, in relation to the technique for forming a groove, shown in PTL 1, some improvement is expected to some extent, but there are some disadvantages in terms of manufacturing costs of the device since an additional structure for prevention is processed, or the like.
On the other hand, in a case where control signals of two systems are required to be fed with the same phase and the same magnitude as in the dual-electrode type MZ optical modulator, it is necessary to use a differential driver and an external phase shifter for driving the light control element and thus the entire device becomes high cost.